This invention relates to pulse tube expanders and, more particularly, to a phase shifter that is highly resistant to plugging.
Some types of sensors and electronic devices are not practically operable at temperatures above about 50-75 K and therefore must be cooled into this temperature range or below for proper operation. The cooling into this midcryogenic range is readily accomplished in some settings, such as a laboratory or a stationary service application, using a reservoir of a cryogenic fluid. In field operations, however, it is often not practical to supply a reservoir of the cryogenic fluid, and thermodynamic-cycle cooling devices are used.
Several types of thermodynamic-cycle cooling devices are known. One such cooler is the pulse tube expander, based upon a modified Stirling cycle, in which pressurized gas in a regenerator/pulse tube assembly is rapidly pulsed such that compression work is done in the warm region of the assembly to remove heat from the expander, and expansion work is done in the cold region to absorb a thermal load. The phase angle between the pressure wave and the mass flow wave is controlled by a small-diameter orifice or a long, small-diameter tube (xe2x80x9cinertance tubexe2x80x9d) located in the gas flow path between a rejection heat exchanger of the pulse tube and a surge tank. Pulse tube refrigeration devices are reasonably efficient, have minimal vibration, are dependable over long service lives, and are of moderate cost. The present invention deals with a cooling system utilizing an improved form of the pulse tube expander.
The present inventors have observed that the operation of the pulse tube expander may be adversely affected by the unavoidable presence of some types of contamination of the working gas. This contamination first reduces the efficiency of the pulse tube expander, and eventually renders it inoperable. There is a need for an understanding of the source of this effect and a solution to the problem of degradation of performance due to the contamination. The present invention fulfills this need, and further provides related advantages.
The present invention provides a pulse tube expander which is fully functional but which has reduced sensitivity to the presence of contamination in the working gas.
In accordance with the invention, a pulse tube expander operable with a pressure source comprises a group of components in gaseous series relation with each other. A regenerator has a regenerator inlet in gaseous communication with the pressure source, and a regenerator outlet. (As used herein, xe2x80x9cgaseous communicationxe2x80x9d includes either a direct gaseous communication or an indirect gaseous communication through an intermediate component such as a heat exchanger.) A pulse tube has a pulse-tube inlet in gaseous communication with the regenerator outlet, and a pulse-tube outlet. A constriction structure that serves as a gaseous phase shifter has a constriction structure inlet in gaseous communication with the pulse-tube outlet, and a constriction structure outlet. The constriction structure comprises at least two independent passageways therethrough extending from the constriction structure inlet to the constriction structure outlet. The constriction structure preferably comprises a porous plug phase shifter with a plurality of independent passageways therethrough extending from the constriction structure inlet to the constriction structure outlet. A surge tank has a surge-tank inlet in gaseous communication with the constriction structure outlet. The pulse tube expander typically includes heat exchangers in the gas flow path, such as a first heat exchanger between the pressure source and the regenerator, a second heat exchanger between the regenerator and the pulse tube, and/or a third heat exchanger between the pulse tube and the constriction structure.
The preferred porous plug constriction structure may take any of a variety of forms. For example, it may be a sintered mass or a packed solid bed of distinct, free-flowing bodies. If the latter, the distinct, free-flowing bodies are preferably non-particulating and non-settling, so that the properties of the porous plug do not change over time. The porous plug is most readily constructed as a tube filled with a porous solid mass. Examples include a tube filled with a plurality of distinct, free-flowing (before placing into the tube) bodies or a tube filled with a plurality of distinct, free-flowing (before placing into the tube) balls. The porous plug sintered mass or free-flowing bodies may be made of any operable material, examples being copper, aluminum, stainless steel, a lead-antimony alloy, glass, or a ceramic.
The sensitivity of the conventional pulse tube expander to contaminants has been traced to the partial or complete blocking of the orifice or the inertance tube by solid matter deposited in this confined cross-sectional area from the gas phase. For some multi-stage applications, the phase shifter operates at cryogenic temperatures. Either solid particulate or condensable gases (such as water vapor) may serve as the contaminant that blocks the orifice or inertance tube in these types of multi-stage applications. The blockage alters the gas flow so that the pulse tube expander loses its calibrated performance or becomes blocked entirely. The inertance tube is more resistant than the orifice to such blockage due to its larger cross-sectional area, but it is also subject to loss of performance and blockage due to contamination.
The multi-path constriction structure of the present invention provides a number of independent passageways extending from the constriction structure inlet to the constriction structure outlet. If one of the passageways becomes partially or fully blocked, gas flow continues in the other passageways. The constriction structure is most conveniently implemented by the use of a porous plug. The porosity may be produced in any operable fashion, and a number of examples are given herein.
The present approach reduces the sensitivity of the pulse tube expander to degradation as a result of contamination by particulates and/or a condensable phase. It is readily implemented to produce the desired constriction performance, with little added weight or size.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.